The present invention relates to methods for chemical process control, and more particularly to methods for determining the strength of a metal processing solution that is capable of spontaneously oxidizing the metal from its elemental state to its ionic state, and thus for determining the etching or stripping rate of the solution.
Etching and stripping solutions are used widely throughout industry to etch or strip unwanted surface layers from metal parts. For example, copper etching solutions, primarily comprising ammonium persulfate, are utilized to strip unwanted copper from masked, copper plated circuit board substrates to produce printed circuit boards. All such solutions function by oxidizing the unwanted metal from its elemental state to its ionic state. As the metal ions build up in the solution, the etching or stripping capability of the solution is decreased in proportion to the increase in concentration of the metal ion. In the past, difficulties have been encountered in quickly and accurately determining when the metal ion concentration has built up sufficiently to render the oxidation rate of the solution too slow to be economically functional.
Heretofore, analytical chemistry techniques have been employed to determine the concentration of the metal ions in processing solutions. Although these techniques are effective and accurate, they suffer from one or more of the drawbacks of being cumbersome, time consuming, or relatively expensive to perform. The amount of time required to monitor the metal ion concentration can be especially quite important. For example, by the time it is determined that a solution has a high metal concentration using conventional analytical techniques, valuable production time that could have been gained by replacing or replenishing the solution has been lost.
It is therefore an object of the present invention to provide methods for quickly and accurately determining the relative strength of metal processing solutions. More particularly, it is an object of the present invention to provide methods that are especially adapted to determine the etching rates of etching and stripping solutions that chemically function to oxidize a metal from its elemental state to its ionic state. Further objects of the present invention are to provide such methods that are simple and economical to perform, that involve little time to complete, that are accurate and reproducible, that can be performed on a production solution at the production site, and that require no particular expertize to provide accurate, reproducible, and easily interpretable results.